The present invention relates to a process for producing a carbonaceous thin plate, and more in detail, relates to a process for producing a carbonaceous thin plate of large in size without occurrence of crevices and cracks therein.
Since carbonaceous materials are excellent in heat-resistance, corrosion-resistance and electric conductivity, application of a carbonaceous material of a thin plate type which is dense and small in permeability to gases, and liquids to separators of phosphoric acid type fuel cells or electrodes for zinc-halogen accumulators has been tried, and the establishment of an industrial process for producing a carbonaceous thin plate of a large type has been desired.
The carbonaceous thin plate according to the present invention may be used for the above-mentioned purposes and other purposes, for instance, electrodes for electrolysis, diaphragms for heat-resistant and corrosion-resistant heat exchangers than the above-mentioned uses.
Hitherto, as a graphite thin plate and a process for producing the same, a graphite thin plate prepared by adding a liquid thermosetting resin to micronized graphite preliminarily treated at a temperature of not less than 2700.degree. C. and of not more than 100 micrometers in particle size, kneading the thus obtained mixture, thereby obtaining a paste, molding the thus obtained paste into a shape of thin plate by a roll-molding machine, hardening the thus molded thin plate by heating or hardening the thus molded thin plate while contacting heated plates with the both sides of the thin plate under a pressure and subjecting the hardened material to the conventional heating and carbonizing treatment and a process of production thereof have been disclosed in Japanese patent application laid-open (KOKAI) No. 59-26907 (1984).
However, in such a graphite thin plate, the adherence of the micronized graphite powders to the thermosetting resin is poor, and voids are apt to be formed in the interface therebetween in the time of subjecting the paste to carbonizing treatment, and it has been regarded difficult to make the gas-permeability of the product less than 1.times.10.sup.-5 cm.sup.2 /s. Namely, such a graphite thin plate has a defect that the thickness of the plate must be large enough in the case where a high impermeability to gases is required.
In Japanese patent application laid-open (KOKAI) No. 59-21512 (1984), disclosed is a process for producing a glass-like carbon thin plate, comprising mixing a chlorinated vinyl chloride resin and/or a vinyl chloride resin with a furan resin, kneading the mixture, molding the thus obtained blend into a film or a sheet-like material and after subjecting the thus obtained molded body to a heat-treatment thereby obtaining a carbon-precursor, and calcinating the thus formed carbon-precursor in an inert atmosphere.
Although the gas-permeability of the thus obtained thin plate is as small as 10.sup.-11 to 10.sup.-10 cm.sup.2 /s (He, .DELTA.P=1 atm), since the yield of carbonization of the resin in the steps of calcination and carbonization is small and the contraction rate of the material is large, it is extremely difficult to produce a thin plate of large size without the occurrence of crevices and cracks and with accurate dimensions. In addition, since the resin as the raw material contains chlorine atoms, the chlorine atoms separate therefrom as strongly corrosive gaseous chlorine or hydrogen chloride, and accordingly, a special consideration for the construction and material of the apparatus is necessary in the case of carrying out the process actually.
In Japanese patent application laid-open (KOKAI) No. 58-150275 (1983), a cell separator of fuel cells comprising a glass-like carbon produced by molding and carbonizing a liquid furan resin, a liquid phenol resin or a mixture thereof is disclosed. Although the gas-permeability of glass-like carbon is extremely small, it takes a long period of time for producing the glass-like carbon and its producibility is poor resulting in a high cost of production.
Although the production of the carbonaceous thin plate has been tried by each of the above-mentioned processes, there is another process wherein a carbonaceous thin plate is produced by carbonizing a carbonaceous raw material, which is carbonized and solidifies after being softened by heating, while applying a pressure thereto by a heating press. Namely, a carbonaceous thin plate is produced by uniformly filling a carbonaceous raw material such as a pitch, etc. into a metal mold of a heating press, heating the carbonaceous raw material and after pressing at a temperature at which the carbonaceous raw material is softened, calcining and carbonizing the carbonaceous raw material at the temperature up to about 600.degree. C. under the pressure, further calcinating or graphitizing the thus carbonized material at a predetermined temperature in a vacuum or in an inert gas.
In the above-mentioned process, the decomposition gas of the carbonaceous raw material generated at the time of carbonization under a pressure by the heating press is scattered and lost to outside through the thin plate or to outside through the interface between the thin plate and the pressing surface, or remains as bubbles within the thin plate. Accordingly, with the increase of the dimensions of the thin plate, it becomes difficult for the decomposition gas of the carbonaceous raw material to scatter to outside. As a result, the decomposition gas remains as bubbles within the thin plate and it becomes impossible to obtain a dense carbonaceous thin plate. In addition, since the volume of the carbonaceous raw material such as a pitch, etc. is reduced by calcination and carbonization, the thin plate apts to contract also in the direction of its surface. With the increase of the dimensions of the thin plate, the friction between the thin plate and the pressing surface of the heating press is increased resulting in impossibility of contraction and finally in occurrence of crevices and cracks in the thin plate.
The above-mentioned phenomena are remarkable in the case where a carbonaceous raw material which has a small yield of carbonization and which undergoes large contaction during carbonization is used.
Accordingly, hitherto a method has been adopted wherein the carbonization under a pressure is slowly carried out for a long time or a method has been taken wherein the carbonaceous raw material is preliminarily subjected to thermal treatment, thereby improving the yield of carbonization and reducing the amount of the decomposition gas at the time of carbonization. However, the above-mentioned processes are insufficient for the purpose.
As a result of the present inventors' studies for solving the problems of the above-mentioned conventional processes, it has been found by the present inventors that in the production of a carbonaceous thin plate by carbonizing a carbonaceous raw material while applying a pressure thereto by a heating press, the scattering of the decomposition gas to outside is facilitated by interposing a paper-like substance which is carbonized without melting between the carbonaceous raw material and the pressing surface of the heating press and that the paper-like substance prevents the adhesion of the carbonaceous raw material to the pressing surface of the heating press, and as a result the carbonacoues thin plate slides easily on the pressing surface thereof, and the occurrence of crevices and cracks due to contraction in carbonization is prevented, and on the basis of those findings, the present inventors have attained the present invention.
Namely, the first object of the present invention is to provide a process for easily producing industrially a dense carbonaceous thin plate of low permeability to gases or liquids and large in size, for instance, each about one meter in length and width without occurrence of crevices and cracks.
The second object of the present invention is to provide a carbonaceous thin plate of large in size not having crevices nor cracks.